Kaposi's sarcoma (KS), once considered a rare tumor largely confined to elderly Mediterranean and African men, has recently re-emerged as the most common neoplasm of patients with the acquired immunodeficiency syndrome (AIDS): 15-25% of such patients will develop this tumor in the course of their human immunodeficiency virus (HIV) infection. While HIV infection is an important risk factor in KS development, studies indicate that it is not sufficient to explain the etiology of the disease. For example, KS is far more prevalent in AIDS patients who acquire their HIV infection by sexual routes than in those who contract HIV by percutaneous inoculation or vertical transmission. These findings suggest that a second, sexually transmitted cofactor may be required for KS development.
Recently, DNA sequences of a novel human herpesvirus (KS-associated herpesvirus, KSHV; also called human herpesvirus 8) have been identified in KS tumors. A growing body of evidence suggests an important role for this virus in KS pathogenesis: (i) infection precedes tumorigenesis and is associated with a striking increase in the risk of subsequent KS development (Moore, et al., 1995) (ii) the distribution of infection among HIV-positive patients parallels known KS risk (Gao, et al., 1996; Kedes, et al., 1996); (iii) all forms of KS, whether HIV-positive or HIV-negative, are strongly associated with KSHV infection (Ambroziak, et al., 1996; Chang, et al., 1994; Chuck, et al., 1996; Huang, et al., 1995; Moore and Chang 1995; Schalling, et al., 1995; Soulier, et al., 1995) and (iv) infection is targeted to the endothelial (spindle) cells, thought to be central to KS pathogenesis (Boshoff, et al., 1995). These findings suggest that prevention or suppression of KSHV infection could reduce the risk of KS development.
The identity of the KSHV protease molecule, activity and inhibitor specificity have been heretofore unknown. The present invention reveals cloning, expression and purification of the KSHV protease (Pr), which, surprisingly, has less than 60% sequence identity with any of the known members of the herpes virus family. Also forming part of the present invention is the characterization of the enzymatic activity of KSHV protease. These discoveries form the basis for screening assays and biochemical substrate inhibitors that can be used to treat Kaposi's Sarcoma according to the treatment methods described herein.